(1) Field of the Invention
This invention concerns methods for converting hazardous compounds found in explosives contaminated soils and specifically, nitroaromatic and nitramine explosives into less hazardous compounds by sequential treatment of the contaminated soil under anaerobic and then aerobic conditions.
(2) Description of the Art
The manufacturing of ordnance has resulted in the contamination of soil and groundwater at munitions production installations with explosives compounds. The nitroaromatic compound 2,4,6-trinitrotoluene (TNT) is one of the most important contaminants found at these installations owing to its wide use, toxicity, and persistence in the environment. Over one million cubic yards of TNT contaminated soil are is known to exist at 30 federal facilities, while substantial quantities may require remediation at thousands of smaller military facilities.
To date, the primary technology used for remediation of explosives contaminated soil at these sites has been incineration. Although incineration is highly effective, it suffers from the disadvantages of high cost and public opposition. Therefore alternative cost effective treatment methods are needed. Bioremediation offers the potential to deliver such an alternative.
The complete mineralization of TNT to carbon dioxide by isolated microorganisms in laboratory-scale experiments has been reported. Fernando, T., J. A. Bumpus, and S. D. Aust. 1990. Biodegradation of TNT (2,4,6-Trinitrotoluene) by Phanerochaete chrysosporium. Appl. Environ. Microbiol. 56:1666-1671. Although mineralization is not the primary fate of TNT in complex natural environments, substantial biotransformation of TNT has been demonstrated to occur in natural environments. For example, TNT is known to be susceptible to reduction of the nitro groups to amino groups via a multi-step process involving nitroso and hydroxylamine intermediates. Walsh, M. E. 1990. Environmental Transformation Products of Nitroaromatics and Nitramines. U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory, Special Report 90-2. It is also known that the predominant metabolite produced under aerobic conditions is 4-amino-2,6-dinitrotoluene (4A26DNT), while under mildly acidic (pH 6) and anaerobic conditions the 2,4-diamino-6-nitrotoluene (24DA6NT) reduction product is most prevalent. Funk, S. B., D. J. Roberts, D. J. Crawford, and R. L. Crawford. 1993. Initial-Phase Optimization for the Bioremediation of Munition-Contaminated Soils. Appl. Envrion. Microbiol. 59:2171-2177. The complete conversion of TNT to 2,4,6-triaminotoluene (TAT) has been reported in samples maintained under strictly anaerobic conditions and low redox potentials (Eh&lt;-200 mV) in the presence of a readily oxidizable carbon source. Lenke, H., B. Wagener, G. Daun, and H.-J. Knackmuss. 1994. TNT-contaminated soil: A Sequential Anaerobic/Aerobic Process for Bioremediation. P.456. Abstracts of the 94.sup.th Annual Meeting of the American Society of Microbiology.
The reduction of TNT to 24DA6NT, 26DA4NT, and TAT is important because aromatic amines can be incorporated into the organic fraction present in soil or compost by forming covalent bonds with a variety of functional groups during the natural process of humification. Major, M. A., W. H. Griest, J. C. Amos and W. G. Palmer. 1996. Evidence for the Chemical Reduction and Binding of TNT during the Composting of Contaminated Soils. U.S. Army Center for Health Promotion and Preventive Medicine Technical Report No. A324278. This binding of aromatic amines into the organic matrix of soils is believed to be irreversible and facilitated by aerobic processes. Aerobic post-treatment may also accelerate the degradation of intermediates produced during anaerobic conditions and speed the consumption of excess carbon which, due to its high oxygen demand, has been reported to cause toxicity to Daphnia magna.
Bioremediation processes that seek to exploit certain aspects of these findings have been developed. The U.S. Army Environmental Center (USAEC), (formerly the U.S. Army Toxic and Hazardous Materials Agency), has developed composting techniques for explosives contaminated soil. This work has led to the field-scale use of composting at the U.S. Army's Umatilla Depot in Hermiston, Oreg. Roy F. Weston, Inc. 1993. Windrow Composting Demonstration for Explosives-Contaminated Soils at the Umatilla Depot Activitiy Hermiston, Oreg. Report No. CETHA-TS-CR-93043. U.S. Army Environmental Center, Aberdeen, Md. Although composting is currently the only technology to have been employed for full-scale cleanup, the USAEC has also sponsored the demonstration of a bioslurry process that cycles between aerobic and anaerobic phases and uses molasses as an oxidizable carbon substrate. Manning, J. F., Jr., R. Boothpathy and E. R. Breyfogle. 1996. Field Demonstration of Slurry Reactor Biotreatment of Explosives-Contaminated Soils. Report No. SFIM-AEC-ET-CR-96178. U.S. Army Environmental Center, Aberdeen, Md.
Despite the advances made in remediating nitroaromatic and nitramine munitions contaminated soils there still remains a need for a soil remediation process that is simple and safe. There is also a need for a process that efficiently converts hazardous nitroaromatic and nitramine contaminants into less hazardous and/or non-hazardous substituents.